Department of Medical Engineering, School of Mechanical Engineering, Shiraz, Fars, Iran.
J Appl Clin Med Phys. 2013 May 6;14(3):4228. doi: 10.1120/jacmp.v14i3.4228.
In the recommendations of Task Group #43 from American Association of Physicists in Medicine (AAPM TG43), methods of brachytherapy source dosimetry are recommended, under full scattering conditions. However, in actual brachytherapy procedures, sources may not be surrounded by full scattering tissue in all directions. Clinical examples include high-dose-rate (HDR) brachytherapy of the breast or low-dose-rate (LDR) brachytherapy of ocular melanoma using eye plaque treatment with 125I and 103Pd. In this work, the impact of the missing tissue on the TG-43-recommended dosimetric parameters of different brachytherapy sources was investigated. The impact of missing tissue on the TG-43-recommended dosimetric parameters of 137Cs, 192Ir, and 103Pd brachytherapy sources was investigated using the MCNP5 Monte Carlo code. These evaluations were performed by placing the sources at different locations inside a 30 × 30 × 30 cm3 cubical water phantom and comparing the results with the values of the source located at the center of the phantom, which is in a full scattering condition. The differences between the thickness of the overlying tissues for different source positions and the thickness of the overlying tissue in full scattering condition is referred to as missing tissue. The results of these investigations indicate that values of the radial dose function and 2D anisotropy function vary as a function of the thickness of missing tissue, only in the direction of the missing tissue. These changes for radial dose function were up to 5%, 11%, and 8% for 137Cs, 192Ir, and 103Pd, respectively. No significant changes are observed for the values of the dose rate constants. In this project, we have demonstrated that the TG-43 dosimetric parameters may only change in the directions of the missing tissue. These results are more practical than the published data by different investigators in which a symmetric effect of the missing tissue on the dosimetric parameters of brachytherapy source are being considered, regardless of the implant geometry in real clinical cases.
在医学物理学家美国协会（AAPM TG43）的 Task Group #43 的建议中，推荐了在完全散射条件下的近距离治疗源剂量学方法。然而，在实际的近距离治疗过程中，源不一定在各个方向都被完全散射的组织所包围。临床实例包括使用 125I 和 103Pd 眼贴治疗的高剂量率（HDR）乳房近距离治疗和低剂量率（LDR）眼黑色素瘤近距离治疗。在这项工作中，研究了缺失组织对不同近距离治疗源 TG-43 推荐剂量学参数的影响。使用 MCNP5 蒙特卡罗代码研究了 137Cs、192Ir 和 103Pd 近距离治疗源缺失组织对 TG-43 推荐剂量学参数的影响。这些评估是通过将源放置在 30×30×30cm3 立方水箱模型的不同位置，并将结果与位于模型中心的源（处于完全散射条件）的数值进行比较来完成的。不同源位置与完全散射条件下的覆盖组织厚度之间的覆盖组织厚度差异称为缺失组织。这些研究的结果表明，径向剂量函数和 2D 各向异性函数的值随缺失组织的厚度而变化，仅在缺失组织的方向上。对于 137Cs、192Ir 和 103Pd，径向剂量函数的这些变化分别高达 5%、11%和 8%。剂量率常数的值没有明显变化。在这个项目中，我们证明了 TG-43 剂量学参数仅在缺失组织的方向上发生变化。这些结果比不同研究者发表的数据更实际，这些数据认为缺失组织对近距离治疗源剂量学参数的对称影响，而不考虑实际临床情况下的植入物几何形状。
